Maternal Immune Activation Drives LINE-1 Viral-Like Mobile DNA Elements That Cause Somatic Mutations in Brain

Background: Growing evidence implicates somatic mosaicism and transposons as sensors of environmental stress in the brain. Exposure to certain environmental factors causes a long-lasting risk for neuropsychiatric disorders and also results in activation of Long Interspersed Nuclear Elements (LINE-1 or L1) retroelements. L1 is a 6kb DNA element with an RNA polymerase II promoter and encodes for the proteins required for retrotransposition. L1 creates somatic mutations during brain development.

Despite the potential for increased L1 activation to cause somatic mutations, the role of L1 in environment associated risk is poorly understood. Recent sequencing of postmortem tissue suggests altered L1-driven somatic mutation profiles occur in Rhett syndrome, schizophrenia and Ataxia-telangiectasia. Studies in mouse and macaque demonstrate that maternal immune activation results in increased L1 activity. Herein, we investigate the functional role of increased somatic retrotransposition in perturbing neuronal development and contributing to neurological disorders.

Methods: We use a combination of in vitro mouse and human neural stem cell differentiation models to interrogate the direct molecular link between Il-6 exposure and L1 activation in neural progenitor cells. We established an in vivo mouse model to manipulate levels of L1 during MIA. Mice exposed to fetal MIA, via polyI:C injection at E9.5, demonstrate impaired sensorimotor gating measured by prepulse inhibition. This parallels the sensorimotor-gating deficits observed in humans at high risk of schizophrenia. We then use a mouse maternal immune activation model of combined with inhibition of retrotransposition by nucleoside analog reverse transcriptase inhibitors to ask if inhibition of L1 reverse transcription has a behavior outcome in sensorimotor-gaiting and open field.

Results: We demonstrate that attenuating L1 activity during fetal MIA specifically ameliorates the sensorimotor gating function without altering the acute pro-inflammatory immune response (n = 8–12 females per group from 3 litters per group, linear mixed model with treatment as a fixed factor and a random intercept for litter, p < 0.01). Interestingly, MIA-related anxiety phenotypes are independent of L1 activity. Using hipocampal neural progenitor cells in vitro, we demonstrate that MIA-induced pro-inflammatory cytokine IL-6 activates transcription and retrotransposition and allows escaping of host surveillance mechanism for L1 elements. We find that exposure of in vitro neural progenitor cells to IL6 leads to a robust activation of L1 RNA (n=5 per group, 1.5 fold, p < 0.001), promoter (n = 5 per group, 20-fold increase p < 0.0001), retrotransposition by reporter assays (n = 5 per group, 2.5fold increase, p < 0.005) and increased accumulation of single-stranded cytoplasmic DNA, which is a result of L1 reverse transcription activity.

Conclusions: This suggests that IL-6 increases L1 activity in neural progenitor cells and that L1 reverse transcription specifically mediates sensory motor gating deficits related to neurodevelopmental disorders caused by maternal immune activation. While L1s remain active in both human and mouse genomes, it is particularly prevalent during neurogenesis and neural differentiation. L1 activity has been thought to contribute to the dynamic genome and expand neuronal diversity and genomic coding potential. Our in vivo experiments combining inhibition of L1 reverse transcription with MIA model during early embryonic development revealed L1 activation as a novel and unexpected regulator of animal behavior.

Disclosure: Nothing to disclose.